rose: check NULL rose_loopback_neigh->loopback

Commit 3b3fd068c56e3fbea30090859216a368398e39bf added NULL check for
`rose_loopback_neigh->dev` in rose_loopback_timer() but omitted to
check rose_loopback_neigh->loopback.

It thus prevents *all* rose connect.

The reason is that a special rose_neigh loopback has a NULL device.

/proc/net/rose_neigh illustrates it via rose_neigh_show() function :
[...]
seq_printf(seq, "%05d %-9s %-4s %3d %3d %3s %3s %3lu %3lu",
rose_neigh->number,
(rose_neigh->loopback) ? "RSLOOP-0" : ax2asc(buf, &rose_neigh->callsign),
rose_neigh->dev ? rose_neigh->dev->name : "???",
rose_neigh->count,

/proc/net/rose_neigh displays special rose_loopback_neigh->loopback as
callsign RSLOOP-0:

addr callsign dev count use mode restart t0 tf digipeaters
00001 RSLOOP-0 ??? 1 2 DCE yes 0 0

By checking rose_loopback_neigh->loopback, rose_rx_call_request() is called
even in case rose_loopback_neigh->dev is NULL. This repairs rose connections.

Verification with rose client application FPAC:

FPAC-Node v 4.1.3 (built Aug 5 2022) for LINUX (help = h)
F6BVP-4 (Commands = ?) : u
Users - AX.25 Level 2 sessions :
Port Callsign Callsign AX.25 state ROSE state NetRom status
axudp F6BVP-5 -> F6BVP-9 Connected Connected ---------

Fixes: 3b3fd068c56e ("rose: Fix Null pointer dereference in rose_send_frame()")
Signed-off-by: Bernard Pidoux <[e-mail]>
Suggested-by: Francois Romieu <[e-mail]>
Cc: Thomas DL9SAU Osterried <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/rose/rose_loopback.c

···
 		}
 
 		if (frametype == ROSE_CALL_REQUEST) {
-			if (!rose_loopback_neigh->dev) {
+			if (!rose_loopback_neigh->dev &&
+			    !rose_loopback_neigh->loopback) {
 				kfree_skb(skb);
 				continue;
 			}